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Physical Pharmacy 21 Colloid: Electrokinetic properties Kausar Ahmad Kulliyyah of Pharmacy

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Presentation on theme: "Physical Pharmacy 21 Colloid: Electrokinetic properties Kausar Ahmad Kulliyyah of Pharmacy"— Presentation transcript:

1 Physical Pharmacy 21 Colloid: Electrokinetic properties Kausar Ahmad Kulliyyah of Pharmacy

2 Physical Pharmacy 22 Contents Types of electrokinetic phenomenaMeasuring zeta potential Traditional microelectrophoresis Laser Doppler velocimetry Applications

3 Electrokinetic phenomena Electrophoresis The movement of charged colloidal particles in electric field. Most practical. Electroosmosis When the charged solid surface is fixed, the electric field causes a movement of the liquid Streaming potential Forcing a liquid through a capillary or porous plug induces a difference of electric potentials Sedimentation potential Forced movement of charged solid particles in a liquid, e.g., due to gravitation induces a difference of electric potentials Physical Pharmacy 23

4 4 Potentials  0 - potential at charged surface  s - potential at Stern layer  - potential at plane of shear. Only zeta potential can be determined experimentally. Both  0 and  s are thermodynamic and theoretical quantities and are calculated from theory only.

5 Zeta potential The slipping/ shear plane separates the thin layer of liquid bound to the solid surface (elastic behavior) from the rest of liquid (normal viscous behavior). The electric potential at the shear plane is called zeta potential. Physical Pharmacy 25 From

6 Determination of zeta potential Measure electrophoretic mobility Measure electrophoretic mobility the electrophoretic mobility is the ratio of the velocity of particles to the field strength Physical Pharmacy 26

7 7 An electrophoresis system consists of a capillary cell with electrodes at either end to which a potential is applied. Observe individual particles using a microscope and time their transit across a graticule. Traditional Measurement of Electrophoretic Mobility

8 Smoluchowski equation From Marian Smoluchowski:   electrophoretic mobility  electric permittivity of the liquid   is the viscosity applies for thin double layer when the zeta potential is not too high large colloidal particles and high ionic strengths Physical Pharmacy 28

9 Huckel Equation size of particle is small compared to EDL (or thick EDL) size of particle is small compared to EDL (or thick EDL) Use Huckel equation: Use Huckel equation:  Physical Pharmacy 29 Thickness of EDL, 1/κ

10 Physical Pharmacy 210 Complications in zeta potential determination Electrophoretic retardation Relaxation effect

11 Physical Pharmacy 211 Laser Doppler Velocimetry Using Fourier Transform, the correlator produces a frequency spectrum from which the mobility distribution and hence zeta potential are calculated. Young’s interference fringes formed at stationary level

12 Fluctuation in intensity of scattered light Physical Pharmacy 212 From:

13 Physical Pharmacy 213 Advantages of laser technique High speed Applicable for non- aqueous, highly conductive, high ionic strength environment Can detect particles < 100 nm Does not differentiate particles i.e. even small ones are detected. Measures over thousands of particles

14 Physical Pharmacy 214 Examples of Application To investigate the electrophoretic properties of blood lipid particles in connection to potential heart problems. Control of size and zeta potential of droplets of artificial blood, which is vital for its safe use. The relationship between the zeta potential of certain cells in amniotic fluid, and lung maturity (suggested by a study in Holland). Determination of IEPs are carried out to e.g. confirm the pH for flocculation/coagulation. Determination of CRC can be used to identify the nature of surface groupings e.g. sulfates, carboxylates etc

15 Physical Pharmacy 215 If pH 8 there is sufficient charge to confer stability. If 4

16 Physical Pharmacy 216 Zeta potential of chloramphenicol and glass particles in benzethonium (+) chloride solution. - an adsorbing cation Charge reversal concentration From: Florence & Attwood

17 Physical Pharmacy 217 References RJ Hunter, Zeta Potential in Colloid Science, Academic Press (1988) RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989) ID Morrison & S Ross, Colloidal Dispersions, Wiley-Interscience, New York (2002)

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